Method and system for non-locking and non-skidding braking/traction of a vehicle wheel

ABSTRACT

System and methods for achieving optimum braking/traction for a vehicle wheel, based upon measurement of road normal and road parallel force on the wheel axle. The measuring signals are continuously recorded in an electronic control unit which undertakes running calculations of the friction coefficient of the wheel against the ground, as well as the variation of the coefficient. The braking force is altered in a continuous manner on the basis of the values of the friction coefficient and the variation thereof, in either an increasing or decreasing direction so that optimum slip for the prevailing conditions is set quickly to achieve optimum braking or traction.

This is a continuation of application Ser. No. 07/582,937, filed Oct. 9,1990, now U.S. Pat .No. 5,195,808.

BACKGROUND OF THE INVENTION

The present invention concerns systems and methods for braking of andtraction for wheeled vehicles, and more particularly optimum braking andtraction under any road or ground conditions.

The principles utilized in the present invention are to a certain extentdescribed in Applicant's previous Norwegian patent application no.88.1513, which hereby is incorporated by reference.

Traction is in the present context meant to be understood as the poweredacceleration of a wheel, i.e. the opposite of braking.

The present invention has particularly been developed in connection withthe need of ability to undertake braking, or possibly traction, whileutilizing maximally the attainable friction coefficient between a wheeland a ground course, particularly with pneumatic tires and the surfaceof runways and roads. Said friction coefficient, usually designated μ,depends on weather conditions, and may therefore vary considerably. Aswill be explained in more detail later in this specification, thefriction coefficient μ also is strongly dependent on the so-called"slip" conditions of the wheel. A precise and fast adjustment to theoptimum braking force value is of considerable benefit for example to apilot in an airplane at departure and landing, because such anadjustment renders possible optimum braking and avoidance of skidding.

For a railway train also the acceleration phase is important, sinceoptimum utilization of power and minimum wear of material is achieved byrapid adjustment of the optimum traction, so that unnecessary anddamaging wheel skidding is avoided and the train reaches its cruisingspeed in the fastest possible manner.

Also for ordinary motor vehicles, i.e. cars, lorries, tractors etc.,good automatic braking systems, i.e. non-blocking systems, are ofinterest for safety reasons. In particular cases, also an effectivetraction control may be advantageous, for example in sports-like drivingand in driving outside roads in rugged terrain.

Braking systems have been developed previously, which systems seek toavoid blocking the wheels during a hard braking action, but it hasturned out that these systems may give results which are not completelysatisfactory, which fact has led to a few airplane accidents in thebraking phase, which accidents might have been avoided. The problem isoften that the existing systems do not really know the state of movementof the airplane, only whether the wheels are rotating or not.

Previously known braking systems for cars are also usually based uponsensing the rotational state of the wheels--if the wheel stops rotating,the braking power is cut. In some related systems, i.e. mainly systemsfor measuring friction, particularly between an airplane wheel and therunway, the friction force is measured for a wheel during braking, at acertain slip factor for the wheel. The slip factor gives an expressionof the slip or the sliding between a rotating wheel and the ground. Itis necessary that a wheel is subjected to slip in order that horizontalforces be transferred when the wheel is rolling. In this connection theslip factor S is defined as ##EQU1## in which n_(k) is the number ofrevolutions of a freely rotating wheel in contact with the ground, andn_(b) is the number of revolutions of the braked wheel.

In the formula above the slip factor is a number between 0 and 1,however said factor may also be expressed in percent, i.e. ##EQU2## andthe slip factor will then be a number between 0 and 100. 100% slip thusmeans a locked, braked wheel (total panic braking), and 0% means nobraking of the same wheel, i.e. a freely rotating wheel.

In airport runways it has become usual to make friction measurementswith a slip factor between 15 and 17%. However, the frictioncoefficients or friction forces obtained in these measurements, willonly be correct, i.e. the maximum possible values, for one particulartype of ground conditions. It has turned out as a fact that the slipfactor which provides maximum runway friction, will be lower under drysummer conditions and higher under slippery winter conditions. In otherwords, a higher slip factor should be set for use under slippery winterconditions than under dry summer conditions.

Non-blocking braking systems comprise means for improving the brakingaction for a wheeled vehicle by providing a reduction of the brakingforce acting on a wheel if said wheel tends to lock or block in a mannerwhich will give starting of skidding after the actual brake application,and said means will thereafter provide a new increase in the brakingforce without necessitating any change in the current braking manoeuver(of a car driver or operator using the brake) which has caused theactual brake application. Such braking systems are advantageous as toreducing the danger of swerving with blocked wheels, and for maintainingthe steering ability during braking, and they may also provide areduction of the braking distances.

When a braking force is supplied to a wheel for reducing the vehiclespeed, a certain percentage of slip is introduced, i.e. the braked wheelrotates slower than the free rolling speed which the wheel would havefor maintaining the instantaneous vehicle speed, and this fact is due tothe friction force between the wheel tire and the ground. When thebraking force is increased, there is also an increase in said frictionforce, and simultaneously the percentage of slip increases until thefriction force reaches a maximum value at a percentage of slip (slipfactor) usually between 10 and 30%, and thereafter the friction forcedecreases with a further increase in the braking force, and at the sametime the slip factor increases to 100%, with locking of the brakedwheel. Still a friction force is present to retard the vehicle (slidingfriction), however this value is lower, and often essentially lower thanthe maximum possible friction force.

The well known expression for the friction coefficient μ is given by

    μ=F/N

in which N is the normal force from the ground on the object lying onthe ground, i.e. in this case the wheel, and F is the friction force. Ifone at first supposes that the normal force N, which ordinarily is equalto or directly proportional to the weight of the object (in the case ofa car, the car weight divided by the number of wheels), is maintainedconstant, it is clear that the friction coefficient μ is directlyproportional to the friction force F. What has been stated aboveregarding variation of the friction force, can therefore in this caseequally well be stated regarding the friction coefficient μ. Thus, μvaries in accordance with the value of the slip factor (see FIG. 1).

From Swedish laid-open publication 394 984 a non-blocking braking systemis known, in which the braking force is controlled in such a manner thatthe wheel supposedly is maintained rolling within the region of maximumfriction force between the tire and the road, i.e. within a region whichpossibly may be more specifically defined and in which region optimumslip percentage is achieved, but the necessary information about theactual slip percentage is not provided in the method according to saidlaid-open publication, paradoxically, nor can any information be foundregarding the forces in question. An electrical control circuit is usedfor detecting wheel rotational states where wheel blocking threatens asa consequence of too powerful braking, together with an electromagnetvalve which is activated via the control circuit for reducing the brakefluid pressure. More specifically, this previously known solution isbased upon utilization of means for providing a DC signal, the amplitudeof which is a function of the rotation speed of the wheel, and no otherphysical parameters than wheel rotation speed are detected. Thus, thesystem does not know how the vehicle moves, and in reality the systemfunctions poorly just in those cases where it is most needed, namelywhen braking the vehicle under slippery conditions, i.e. with typicallylow values of the friction coefficient. A problem will also exist at lowspeeds, since the measuring signal in this case will not be very useful.The problem is, as previously mentioned, that irrespective of electronic"smartness" as to use of reference values for maximum allowedretardation of wheel rotation, the basic information about how thevehicle is really moving, is missing.

From Swedish laid-open publication 413 082 there is known aslide-preventing control device for braking a vehicle. Also in this casethe aim is preferentially to maintain the slip within a region which iscoordinated with a maximum friction coefficient. Periodic modulation ofthe wheel torque is used together with wheel acceleration measurementsin order to determine the direction of change of the frictioncoefficient from an optimum value as a function of slip, while anintegral/proportional control of the pulse modulation is supposed torender possible a compensated variation of the wheel torque and the slipstate into a state providing optimum friction force. The control devicemay be utilized for preventing skidding of the vehicle not merely duringbraking, but also during acceleration. However, this system has the sameweaknesses as the previously mentioned system, since merely the wheelrotation and the variation thereof is detected.

In contrast hereto, there is known from Swedish laid-open publication382 781 a system for preventing blocking of a vehicle wheel, whichsystem lies closer to the system dealt with in the present application.In SE 382 781 one measures the horizontally acting reaction force on thewheel axle, which reaction force is proportional to the friction forcebetween the wheel and the ground, and a signal representing the measuredroad parallel force, is used for controlling the braking power in such adirection that the friction force is maximized. This system can only beutilized in connection with braking, and the system does not take intoaccount variations in the road normal force N. It should at this pointbe emphasized that particularly during braking or acceleration phasesthe reaction force will cause a fast change of the road normal force forthe individual wheels. When N varies, F, that is the road parallelforce, may experience a necessary change due to the change in N. Theknown system will misinterpret such a situation, and believe that thechange in F is due to applying an incorrect braking power, even thoughthe instantaneous braking power is actually correct. Possibly thedirection of the immediately necessary change of braking power may bemisinterpreted when F is influenced by a change in N. Thus, the knownsystem will be "fooled" in a situation like this, and will therefore usemore time for adjusting to the correct slip value than what is optimum.

SUMMARY OF THE INVENTION

The present invention aims to improve the last mentioned system, byproviding methods and devices for achieving optimum braking/traction ofa wheeled vehicle, by measuring both road parallel and road normalforces and using runningly calculated friction values for optimizing thebraking power. Using the invention, auto-tracking toward optimum brakingor traction force is achieved on any ground surface and in anysituation.

The invention is defined precisely by means of the enclosed patentclaims.

Thus, using the systems/methods in accordance with the presentinvention, that slip factor is sought which provides maximum frictioncoefficient. As long as the normal force N stays constant, maximizingthe friction coefficient μ will be exactly the same as maximizing theroad parallel force or the friction force F. But if the road normalforce is changed abruptly, namely just during braking or acceleration, acomputer calculation based upon measurement of both road parallel androad normal force, and including a calculation of the frictioncoefficient μ as well as the variation thereof, adjusts into a state ofcorrect slip value under existing road and weather conditions fasterthan the previously known solution according to SE 382 781.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall be described in more detail below, referring to thedrawing figures, in which:

FIG. 1 shows typical friction coefficient/slip curves for different roadconditions, as well as an indication of the variation region for optimumslip/maximum braking effekt,

FIG. 2 shows in a purely schematic manner signal circuits in connectionwith a vehicle wheel,

FIG. 3 shows a schematic diagram of a traction system,

FIG. 4 shows an example of mounting of force measurement devices in acar chassis, and

FIG. 5 shows correspondingly an example of mounting of force measuringdevices in an airplane undercarriage.

DESCRIPTION OF PREFERRED EMBODIMENTS

The ability to stp or accelerate a vehicle is principally determined bythe characteristics of the contact area between the road surface and thewheel surface, and these characteristics are often described by statingthe friction coefficient μ, which is defined as the ratio between thefriction force opposing the motion between the two surfaces and theforce between the surfaces perpendicular thereto, in accordance with themathematical expression defined previously in the description.Concerning rolling surfaces, μ is furthermore a function of the slipparameter, which has also been defined above.

It is previously known that the variation of the friction coefficientdepending on slip, for many different sorts of road surfaces and wheeltires or wheel surfaces generally follow such measurement curves asdisplayed in FIG. 1, in which μ is shown as a function of slip. Animportant characteristic of these curves, is that μ in each case has amaximum value. The particular shape of the curve is without importance,since the shape may vary within wide limits according to the conditions,inter alia the speed. The fact that a maximum value exists, entails thatthe skid-preventing braking system preferably should function in such amanner that braking occurs at just this maximum value of the frictioncoefficient, if the shortest possible braking distance (maximum brakingability) is desirable, respectively in such a manner that traction isexercised at such a maximum value if the best possible acceleration ofthe vehicle is desirable.

In FIG. 1 curve 1 relates to a typical pneumatic rubber wheel against adry and firm road surface, giving typically high μ values, for exampleμ_(max) =0.9, and the remaining curves 2-7 relate to the same pneumaticrubber wheel upon successively "worse roads", curve 7 typically relatingto winter road conditions of a relatively slippery type, with a maximumfriction coefficient μ_(max) =0.15. It should be noted that the regionof optimum slip is located differently for the different types of roadsurface or road "conditions". For curve 1 optimum slip is located atabout 7,5%, for curve 2 optimum slip is found at about 15%. while forcurve 7 a slip of about 35% should be preferred in order to achieve thebest possible retardation effect under these conditions. An indicationof the optimum slip region is shown by means of the dashed lines in FIG.1.

In other words: If the shortest possible braking distance, or possiblythe best possible traction/acceleration is desirable, then the brakingforce/traction power must be controlled in such a manner that the wheelrolls inside the region of maximum friction force between the wheelsurface and the road surface, i.e. within the region for achievingoptimum slip.

It should be noted that for railway locomotives, with steel wheelsresting upon rails, the optimum slip region is located at about 2-5%under normal conditions, i.e. rather far to the left in the diagram asshown in FIG. 1. An anti-skid system which works well, will be of greatvalue in a case like this, in which uncontrolled skidding often willappear when using manual methods, both in traction and braking.Traditional wheel rotation sensors also function very poorly when atrain is in the starting phase, with a very low wheel rotation speed. Asystem like the one according to the present invention is able toprovide the necessary control of both the traction and the brakingphase.

In FIG. 2 is shown the general principle for the braking system inaccordance with the invention. The wheel on the surface 2 has an axis 3which represents a wheel axle. Two load cells, 4, 5 are mounted formeasuring respectively horizontal reaction force in the wheel axlecorresponding mainly to the current friction force F between the wheelsurface and the ground surface, and the vertical reaction force in thewheel axle, which force mainly corresponds to the vertical force N fromthe road surface 2 against the wheel 1. In parenthesis it should benoted that with a view to driving i slanting terrain, in the followingthe expressions "horizontal" and "vertical" will be substituted by "roadparallel" and "road normal" concerning the forces in question whichoccur in the engangment point between wheel 1 and road surface 2.

The measuring signals from the two load cells 4 and 5 are passed to acomputer 6 of microprocessor type, which in principle carries out thefollowing operations:

a) Successively and with short intervals, the current frictioncoefficient is calculated in accordance with the expression

    μ=F/N

in which F and N symbolically represent the signals from thecorresponding load cells 4 and 5. If the load cells should happen to benon-linear, the computer is of course equipped with either a correctioncircuitry near the input, or a programmed correction processing of thesesignals before calculating the friction coefficient.

b) The current friction coefficient is compared with the previousfriction coefficient.

c) In accordance with a well defined program, which program it is notnecessary to describe in detail in this specification, the computer 6then controls a proportional valve 7 which regulates the power from ahydraulic pump unit 8 to the disc brake 9. The program strategy is asfollows: As long as a calculated μ is larger than the previous one, thebraking power is increased with a predetermined or empiricallydetermined increment. As soon as μ starts to decrease, the braking poweris lowered quickly using a somewhat larger increment, which is alsopredetermined or empirically determined, and thereafter the brakingpower again increases successively, and a cycle as described above,restarts. In this manner there is achieved a rapid hunting of and"commuting around" the summit in question for a frictioncoefficient/slip curve, i.e. within the indicated optimum region shownin FIG. 1, and the braking effect will be maintained in the optimumregion under the given conditions.

The same type of control is achieved in a system for traction. It isreferred to FIG. 3 which shows an example of a traction system inaccordance with the present invention. FIG. 3 displays a system for afour wheel vehicle with separate operation of all four wheels 1. In theshown case each wheel 1 is monitored by two load cells 4, 5, or possiblya load cell with double action, as will be explained later, and all loadcells are connected to the computer or the electronic control unit 6. Asmentioned previously, each wheel 1 is separately operated by its ownservo-motor 10, the torque of which is controlled by separate pressureregulators 11 for the pressure from a main motor 12, and said pressureregulators 11 receive their power signals from the electronic controlunit 6 on the basis of corresponding current calculations as describedin the braking case above. One has contemplated at this point the systemmounted in an all-terrain 4-wheel drive vehicle, in which optimumtraction in any situation and for each wheel separately is of greatimportance. Of course, nothing prevents that the number of separatedriving motors and separate force measurement devices for the roadparallel and the road normal force are adapted for the particular case.It is of course possible to combine two wheels on one axle, possiblyonly make measurements for one single of all the wheels, etc. However, asystem like the one displayed in FIG. 3 will be an optimum system, sincethe road or ground conditions in principle may vary from one wheel toanother.

Previously in this specification there was mention of the possibility ofutilizing "double action" force measurement devices or load cells. It isof course a fact than when two orthogonally directed forces are to bemeasured simultaneously, it will be possible to measure their resultantin a tilted direction. Thus, in particular cases it may be favourable toarrange only one single force measuring device, so that the signal fromthis device to the electronic control unit is pre-processed in order tofind the two components in question, i.e. the road parallel F and roadnormal N. To be able to execute the calculation, of course the angle ofthe measured force in relation to e.g. the road parallel direction mustbe known. If this angle is known to be stable, the conversion may beexecuted in a simple manner using simple trigonometric relations. Incertain cases, displacements or torsions of those materials which theload cells are mounted in or on, must be taken into consideration, andit may then be necessary to utilize a separate angle sensor which isalso connected to the electronic unit so that the correct angularrelations can be considered during the calculations. However, thereexists also a third possibility, namely that the torsion anddisplacement conditions are investigated experimentally in a productionphase, so that the electronic control unit may be programmed forexecuting compensating angular calculations on the basis of the forcemeasurements themselves.

Examples of load cell mounting are shown in FIG. 4 and FIG. 5. In FIG. 4a typical front wheel chassis of a car is displayed, and in such a caseload cells 5 for measuring road parallel forces, as indicated by arrowsin the drawing, may be mounted for example in a force-absorbing bar 13in a position where the bar is normally fixed to the car body, foreffective measurement of the reaction force between the body, i.e. theremaining mass of the car, and the wheel suspension. Of course thesituation will often be that several elements absorb parts of e.g. thetotal road parallel force. Thus, the measured force in question canoften have a certain and known relation to the total force, and it is ofcourse possible to compensate for this in the electronic control unit 6.Obviously the same holds valid for the road normal force, and suchscaling relations can be set and programmed both calculationwise andempirically. Furthermore the situation will be that even when usingseparate load cells for road parallel, respectively road normal forcefor a wheel, the current forces or torques which are measures, may betilted or have certain relations to the actual normally directed orparallel directed force. Where and how the load cells are arranged, willthus vary in accordance with the rest of the vehicle construction.

In certain cases it may be established that during braking or traction,there will not be any considerable variation of the normal force N forthe individual wheel. In such a situation it is of course possible touse the simplification which is to leave out measurement of the roadnormal force N, letting the road parallel force F itself replace thefriction coefficient μ in the calculations of the electronic controlunit.

In FIG. 5 there is shown a suitable location for a load cell formeasuring the road normal force, or a tilted force from which the roadnormal force N may be calculated, in an airplane wheel undercarriage.The slanted bar 14 transmits forces which are closely related to theroad normal forces in question. In joint 15 is situated a bolt which maybe modified in a simple manner in order to comprise a load cell fortransmitting a signal to the computer 6 of the braking system. Formeasuring the road parallel forces on the wheels, force or torquemeasuring devices may be utilized, which are mounted in close proximityto the wheel axles, e.g. in the area indicated with reference numeral16.

Suitable load cells may have many different shapes. For example, straingauges, semi-conductor force measuring devices or piezo-electric sensorsof per se known types may by used. The construction, shape andadaptation of the particular sensor type often must be tried out and"tailored" for the constructions in question, but this side of thematter constitutes an ordinary technical problem to be solved, and doesnot in itself constitute any part of the present invention.

It is self-evident that in many situations it is not desirable to usethe effective, automatic braking or traction system, since for examplethe braking system used under normal conditions implies a rather hardbraking experience, which often will be perceived as unpleasant. Undergood friction conditions a braking retardation of 0,5-0,6 G is oftenachieved, and such a braking is felt as rather dramatic. Therefore,under good conditions and with a long and surveyable braking distance,one will prefer to use the vehicle's brakes in a "manual" manner, slowlyand easily. Therefore, in a sensible braking or traction systemprecautions must be taken to ensure that the system only takes effectwhen this is actually required. The first obvious possibility is ofcourse the well known "panic braking" in which an unforeseen situationturns up, bringing about a rapid and hard employment of the vehiclebrake. In such a case the automatic braking system must of course takeover, and for this reason it will be appropriate with a sensor inconnection with the driver's or operator's operating device, usually abraking pedal. Such a sensor may respond to either the deflection of theoperating device, or the velocity of the operating device. The computer6 is then provided with stored limiting values for the deflection orvelocity of the operating device, and a comparison is made of thecurrent value and the limiting value. When a limiting value is exceeded,the braking system is activated. Another obvious possibility is thate.g. the pilot in an airplane has been informed in beforehand that thelanding conditions are difficult, or possibly he may see that theaccessible landing runway is shorter than desirable, and he may thenorder activation of the braking system in advance by pushing a button.Situations may also be contemplated in which for example a "dead man'scontrol" is triggered in a train or some other vehicle, and thereby bothtriggers the braking power generally as well as the optimum brakingsystem in accordance with the present invention. Possibly other types ofemergency situations may be detected and automatically trigger a brakingprocedure which includes utilization of the optimum braking system inaccordance with the present invention.

Obviously it is possible to combine in one and the same vehicle botheffective traction and effective braking by "double" utilization of thepresent invention. One and the same computer 6 is used, and the same setof force or torque measuring devices 4, 5 are used in the combination ofa traction and a braking system. The computer 6 will, depending on thestate of driving, which is detectable quite simply by sensing thedirection of the road parallel force F, respectively activate controldevices for motors in a traction phase and control devices for brakingpower in a braking phase. For example all-terrain vehicles may benefitgreatly from such a combination variant of the present invention.

In certain cases the braking executing organs and the controllable motororgans may be constituted by one and the same system, e.g. by usingelectromotors/generators, which during traction/ordinary driving operateas motors, but during braking conveys energy back to the energyreservoir or the vehicle (electrochemical batteries or other types ofreservoir, for instance of the flywheel type) by operating asgenerators. Such cases may be particularly well adapted for combinationwith the above mentioned combined traction/braking control system inaccordance with the present invention.

Once again it must be underlined that the present control principle forachieving effective traction or braking, in contrast to most previouslyknown systems, are based upon the measurement of forces, and not uponmeasuring the rotation speed of the wheel. The wheel rpm is notnecessary in order that the present invention shall function in asatisfactory manner. Nevertheless, in some cases it may be favourable toinclude the further information that may be achieved by sensing thewheel rotation speed, so that a further combination effect is achieved.

The ordinary "ABS" brakes used to-day, have the following unfortunateproperties:

a) The ABS system is dependent on a certain minimum speed to functionproperly, because the wheel rotation speed is sensed directly.

b) The ABS system is partly dependent on measurements being undertakenon one or several other wheels in order to function properly.

c) Blocking of brakes are prevented, but the braking distance may beextended.

d) As a consequence of item a) above, the ABS system is unreliable atlow speeds.

e) An ABS system renders no possibility for displaying the currentfriction conditions to a vehicle driver, for example an airplane pilot.

f) The ABS systems function at their best at high values of the frictioncoefficient. Because the optimum slip is totally different underslippery driving conditions, the ABS systems are not optimum under suchconditions.

For the present invention the corresponding items hold valid:

a) It is not necessary to measure the wheel rotation speed.

b) If it is desired, each wheel may operate and be measuredindependently.

c) The braking distance is reduced essentially in all existing road andweather conditions, and at all existing speeds.

d) The system in accordance with the present invention is exactly asreliable at low speeds as at high speeds.

e) It is possible to present values of e.g. the friction coefficient ina display with the vehicle driver, in such a manner that informationactually is given regarding the current braking conditions. In thisrespect a continuous electronic monitoring of the road conditions isactually undertaken by means of the present system.

f) It is actually a fact that when using a braking system in accordancewith the present invention, the electronic circuitry functions at itsbest in the case of typically "flat" friction coefficient curves (seeFIG. 1, curve 7) which is typically found in connection with a slipperyroad surface. In other words, the present system functions well exactlyin those situations where achieving optimum braking is important.

I claim:
 1. A method for achieving optimum braking of a vehicle wheelhaving an axle and a ground surface interface, comprising measuringcontinuously forces acting on the wheel axle, including a road normalforce F_(norm) perpendicular to a ground surface on which the wheeltravels and a road parallel force F_(par) parallel to said surface, bymeans of at least one force measuring device,wherein braking power iscontrolled by means of an electronic control unit in accordance withresults of running calculations based upon the force measurements, saidforce measuring devices being connected to said control unit, and anapproximate braking ratio F_(par) /F_(norm) as well as a variationthereof are determined continuously in the control unit from themeasured force values during a braking procedure where braking power isincreased or decreased in a continuous manner on the basis of saidapproximate braking ratio F_(par) /F_(norm) and the variation thereof,to obtain a maximum instantaneous braking ratio, said maximuminstantaneous braking ratio being substantially equal to aninstantaneous friction coefficient μ for the instant ground surfaceinterface of said wheel.
 2. A method in accordance with claim 1, whereinsaid braking power is only influenced by the electronic control unit ifan optimum braking effect is necessary.
 3. A braking system for awheeled vehicle comprising operator's brake operating devices, brakecommand transmitting means for transmitting a brake command from saidoperating devices to a braking power supply means to which is alsoconnected an electronic control unit, and braking exercising meansattached to the vehicle wheels, said braking power supply means beingoperatively connected to said braking exercising means;furthercomprising at least one force measuring device attached to an axle of atleast one of the vehicle wheels for measuring continuously forces onsaid axle comprising a road normal force F_(norm) perpendicular to asurface on which the vehicle is to travel and a road parallel forceF_(par) parallel to said surface, and being connected to said electroniccontrol unit for signal delivery, wherein said electronic control unitcomprises means to increase or decrease braking power from said brakingpower supply means to said braking exercising means during a brakingprocedure, in accordance with results of continuous calculations of anapproximate braking ratio F_(par) /F_(norm) as well as a variationthereof, on the basis of measured force values delivered to theelectronic control unit from said force measuring devices, to obtain amaximum instantaneous braking ratio, said maximum instantaneous brakingratio being substantially equal to an instantaneous friction coefficientμ for the instant wheel to ground interface.
 4. A method for achievingoptimum propulsive traction for a vehicle wheel having an axle and aground surface interface, comprising:measuring continuously forcesacting on the wheel axle, including a road normal force F_(norm)perpendicular to a ground surface on which the wheel travels and a roadparallel force F_(par) parallel to said surface, by means of at leastone force measuring device, wherein propulsive traction power iscontrolled by means of an electronic control unit in accordance withresults of running calculations based upon the force measurements, saidforce measuring devices being connected to said control unit, and anapproximate propulsion ration F_(par) /F_(norm) as well as a variationthereof are determined continuously in the control unit from themeasured force values during a propulsion procedure where propulsionpower is increased or decreased in a continuous manner on the basis ofsaid approximate propulsion ratio F_(par) /F_(norm) and the variationthereof, to obtain a maximum instantaneous propulsion ratio, saidmaximum instantaneous propulsion ratio being substantially equal to aninstantaneous friction coefficient μ for the instant ground surfaceinterface of said wheel.
 5. A method in accordance with claim 4, whereinthe traction power is only influenced by the electronic control unit ifoptimum traction effect is necessary.
 6. A propulsive traction systemfor a wheeled vehicle comprising operator's propulsive tractionoperating devices, propulsion command transmitting means fortransmitting a propulsion command from said operating devices to apropulsion power supply means to which is also connected an electroniccontrol unit, and propulsion exercising means attached to the vehiclewheels, said propulsion power supply means being operatively connectedto said propulsion exercising means;further comprising at least oneforce measuring device attached to an axle of at least one of thevehicle wheels for measuring continuously forces on said axle comprisinga road normal force F_(norm) perpendicular to a surface on which thevehicle is to travel and a road parallel force F_(par) parallel to saidsurface, and being connected to said electronic control unit for signaldelivery, wherein said electronic control unit has means to increase ordecrease braking propulsive power from said propulsion power supplymeans to said propulsion exercising means during a propulsion procedure,in accordance with results of continuous calculations of an approximatepropulsion ratio F_(par) /F_(norm) as well as a variation thereof, onthe basis of measured force values delivered to the electronic controlunit from said force measuring device, to obtain a maximum instantaneouspropulsion ratio, said maximum instantaneous propulsion ratio beingsubstantially equal to an instantaneous friction coefficient μ for theinstant wheel to ground interface.
 7. A method for achieving bothoptimum propulsive traction force and optimum braking of a vehicle wheelhaving an axle and a ground surface interface, comprising:measuringcontinuously forces acting on the wheel axle, including a road normalforce F_(norm) perpendicular to a ground surface on which the wheeltravels and a road parallel force F_(par) parallel to said surface, bymeans of at least one force measuring device, wherein propulsivetraction power and braking power are controlled by means of anelectronic control unit in accordance with results of runningcalculations based upon the force measurements, said force measuringdevices being connected to said control unit, and a force ratio F_(par)/F_(norm) as well as a variation thereof being determined continuouslyin the control unit from the measured force values during a vehiclespeed change procedure where one of the propulsive traction power andthe braking power is increased or decreased in a continuous manner onthe basis of said force ratio F_(par) /F.sub. norm and the variationthereof, to obtain a maximum instantaneous force ratio, said maximuminstantaneous force ratio being approximately equal to an instantaneousfriction coefficient μ for the instant ground surface interface of saidwheel.